Many conventional wireless communication systems employ either omnidirectional or low-directivity antennas at both the base station and the subscriber stations primarily because of the comparatively long wavelength of the frequencies used. For example, some wireless local area networks use frequencies ranging from about 2.4-5 gigahertz (GHz), which have wavelengths ranging between 6 and 12 centimeters (cm). Directional antennas could improve the throughput of these systems, but the longer wavelengths of the signals make compact directional antennas difficult to implement. Furthermore, the propagation properties of these longer wavelength signals result in a rich multi-path indoor environment which allows multi-antenna multicarrier modulation techniques, such as multiple-input, multiple-output (MIMO) OFDM, to provide reliable coverage, negating any need for directional antennas.
The millimeter-wave band, however, may have available spectrum capable of providing even higher-level throughputs. For example, throughputs of up to several gigabits per second (Gbps) or more may be possible. One issue with using millimeter-wave frequencies for communicating is that millimeter-wave frequencies are easily absorbed by the atmosphere and objects, including humans, wasting a significant portion of their energy. Another issue with using millimeter-wave frequencies for communicating is shadowing, because millimeter-waves generally do not travel around objects. Shadowing makes communicating more difficult in non-line of site (NLOS) situations.
Thus, there are general needs for communications systems and methods for communicating within the millimeter-wave frequency band with greater throughput. There are general needs for communications systems and methods for communicating within the millimeter-wave frequency band that waste less energy and/or mitigate the effects of shadowing.